87
as to their nutritional mode because there is no observable host response. Moreover,
larger sample sets of specimens displaying consistent patterns of such associations are
relatively rare. Spectacular exceptions include one particular type of glomoid spore
located in degrading land plant axes that contains evidence of (simultaneous) colonization by three different intrusive fungi (Krings and Harper 2018). In this study, the
authors report massive callosities that occur around the intrusion filaments of a chytrid-like parasite with epibiotic sporangia, while at the same time the penetration
hyphae of another fungus can extend into the same spore without triggering a host
response. Moreover, several of the spores show large numbers of short, inwardly
directed projections, each consisting of a short hyphal branch of a third fungus encased
in a callosity of host wall material (Fig. 3.4c). It should be noted that it is exceedingly
rare to find the causative agent preserved in direct contact with the callosity (or any
host response); typically, only evidence of the host response is found (Fig. 3.4d).
There are several reports of fungal mycelia and reproductive units occurring on
the surface of or within other fungi also from geologically younger deposits (e.g.,
White and Taylor 1989: pl. 2, figs 1, 2, 6; 1991: pl. II, figs 1 and 3, pl. III, figs 1–5;
Taylor et al. 1994: pl. I, fig. 2; Taylor et al. 2005c: fig. 7; Krings et al. 2011c: fig. 1L;
Krings and Taylor 2012: pl. I, fig. 4; Harper et al. 2015: pl. 1, figs 13 and 14; White
et al. 2018: pl. IV, figs 3–6). However, as with the Rhynie chert specimens, there is
mostly no direct evidence of parasitism in the form of a host response.
3.3.3.2 Fossil Fungal “Sporocarps”
One group of fungal fossils that have received considerable attention are the socalled “sporocarps”, enigmatic structures, mostly from the Carboniferous and
Triassic, that are composed of a walled cavity enveloped in an investment of interlacing or tightly compacted hyphae (surveyed in Krings et al. 2013; Taylor et al.
2015). In some specimens, the cavity is empty, but more often contains one to several spheres that have been the basis for several hypotheses regarding the affinities
of these structures. One interprets them as cleistothecia and suggests affinities with
the Ascomycota based on specimens containing one large sphere believed to represent an ascus that in turn contains several smaller spheres interpreted as ascospores (Stubblefield and Taylor 1983; Stubblefield et al. 1983). An alternative
interpretation views the large sphere as a zygospore, and the entire structure accordingly as a reproductive structure (i.e. a mantled zygosporangium) of a member of
the zygomycetous fungi (Pirozynski 1976; Taylor and White 1989). If this latter
interpretation is accurate, then the smaller spheres found within the large sphere in
some specimens would represent some type of mycoparasite. There is an increasing
body of circumstantial evidence to corroborate the latter hypothesis. For example, a
“sporocarp” specimen from the Carboniferous of Great Britain contains not only
spherical structures, but also hyphae forming appressorium-like swellings at the
contact region with host walls (Fig. 3.4e, f) (Krings et al. 2011b).
3 Fungi as Parasites: A Conspectus of the Fossil Record
as to their nutritional mode because there is no observable host response. Moreover,
larger sample sets of specimens displaying consistent patterns of such associations are
relatively rare. Spectacular exceptions include one particular type of glomoid spore
located in degrading land plant axes that contains evidence of (simultaneous) colonization by three different intrusive fungi (Krings and Harper 2018). In this study, the
authors report massive callosities that occur around the intrusion filaments of a chytrid-like parasite with epibiotic sporangia, while at the same time the penetration
hyphae of another fungus can extend into the same spore without triggering a host
response. Moreover, several of the spores show large numbers of short, inwardly
directed projections, each consisting of a short hyphal branch of a third fungus encased
in a callosity of host wall material (Fig. 3.4c). It should be noted that it is exceedingly
rare to find the causative agent preserved in direct contact with the callosity (or any
host response); typically, only evidence of the host response is found (Fig. 3.4d).
There are several reports of fungal mycelia and reproductive units occurring on
the surface of or within other fungi also from geologically younger deposits (e.g.,
White and Taylor 1989: pl. 2, figs 1, 2, 6; 1991: pl. II, figs 1 and 3, pl. III, figs 1–5;
Taylor et al. 1994: pl. I, fig. 2; Taylor et al. 2005c: fig. 7; Krings et al. 2011c: fig. 1L;
Krings and Taylor 2012: pl. I, fig. 4; Harper et al. 2015: pl. 1, figs 13 and 14; White
et al. 2018: pl. IV, figs 3–6). However, as with the Rhynie chert specimens, there is
mostly no direct evidence of parasitism in the form of a host response.
3.3.3.2 Fossil Fungal “Sporocarps”
One group of fungal fossils that have received considerable attention are the socalled “sporocarps”, enigmatic structures, mostly from the Carboniferous and
Triassic, that are composed of a walled cavity enveloped in an investment of interlacing or tightly compacted hyphae (surveyed in Krings et al. 2013; Taylor et al.
2015). In some specimens, the cavity is empty, but more often contains one to several spheres that have been the basis for several hypotheses regarding the affinities
of these structures. One interprets them as cleistothecia and suggests affinities with
the Ascomycota based on specimens containing one large sphere believed to represent an ascus that in turn contains several smaller spheres interpreted as ascospores (Stubblefield and Taylor 1983; Stubblefield et al. 1983). An alternative
interpretation views the large sphere as a zygospore, and the entire structure accordingly as a reproductive structure (i.e. a mantled zygosporangium) of a member of
the zygomycetous fungi (Pirozynski 1976; Taylor and White 1989). If this latter
interpretation is accurate, then the smaller spheres found within the large sphere in
some specimens would represent some type of mycoparasite. There is an increasing
body of circumstantial evidence to corroborate the latter hypothesis. For example, a
“sporocarp” specimen from the Carboniferous of Great Britain contains not only
spherical structures, but also hyphae forming appressorium-like swellings at the
contact region with host walls (Fig. 3.4e, f) (Krings et al. 2011b).
3 Fungi as Parasites: A Conspectus of the Fossil Record